Porous materials, metallic, ceramic and polymeric, have widespread applications. For different applications however different pore sizes and porosities are needed. A spherical pore geometry is often preferred since this offers better mechanical properties and offers a larger thermal shock resistance in high temperature applications. Production of these materials in a controlled manner is an important aspect of this technology. Some production methods in the art apply various colloidal processing routes comprising methods such as sacrificial materials, partial sintering and gas inclusion, as detailed below.
The technology will be explained with reference to a green body, i.e. a non-sintered material, in particular a non-sintered ceramic or metallic material. Green bodies have been for instance described in U.S. Pat. No. 6,596,799. A green body is a powder packing which has the ability to retain its shape. The green strength is the amount of force a green body can sustain before the shape retention is lost. Green bodies may be prepared from suspensions, pastes or dry powder of either ceramic powders, such as various kinds of ferrites, titanates, alumina, cordierite, titanium dioxide, silicon carbide, silicon nitride and the like, or metallic powders, such as steel, titanium and its alloys, aluminium and its alloys, by processes such as pressing of granules, slip casting and extrusion. Methods for obtaining manufacturing a ceramic or metallic green body for ceramic or metallic slurries containing a ceramic or metallic powder are well described in the art.
Sacrificial materials: a common route to obtain pores is the inclusion of an organic material into a green body. These organic materials can have almost any shape and/or size, and a multitude of such materials, including starch and polymers may be used. Mostly spherical particles are used since spherical pores offer obvious advantages in terms of mechanical and thermal shock properties. However, the organic material particles have to be removed for instance by burning out during a thermal step, called the de-binding step, before sintering can be done.
De-binding is a major step in the production of porous structures from metallurgic powders of the prior art. In common manufacturing processes of powder metallurgy, in order to facilitate ceramic powder to cast into bodies more easily, polymer materials are frequently added to the bodies as cast additives. Such type of cast additives includes adhesives, surfactants, fillings or lubricants. The cast additives are mixed with polymer materials for casting bodies that may be formed by methods such as moulding, forging, extrusion, injection or scraping. The formed bodies are generally placed into furnaces for de-binding (removal of the organic materials such as the casting polymers) as the next step. However, the cast additives used come as high as 30% by volume, and defects incurred are prone to arise during removal of the polymer materials in the de-binding process. De-binding processes currently used include solvent de-binding and thermal de-binding, as follows:    a) Solvent de-binding is implemented by the steps of dipping a body into a solvent, and extracting dissolvable adhesives, fillings, surfactants or lubricants from the body for forming successive openings penetrating from an interior to an exterior of a sample. Thus, subsequent heating is able to facilitate exsudation of residual adhesive in form of a gas or a liquid through the openings. In addition, de-binding efficiency is increased while decreasing defects by pressurising the solvent to a supercritical state or heating the solvent into steams. However, such means of solvent de-binding brings forth environmental and recycling issues and thus further increases processing expenses thereof.
Thermal de-binding is implemented by the steps of placing a body into a furnace, and removing adhesives directly or after solvent de-binding. Only human-friendly gases that give no environmental, recycling or human-hazardous issues like those in solvent de-binding are produced, and therefore thermal de-binding is the most extensively applied de-binding process. Nevertheless, it is necessary to pre-heat the furnace to a temperature required for thermal de-binding, meaning that time and energy for pre-heating and energy consumed during maintaining heat are costly, and an efficiency problem often abstained by the manufacturing process is resulted. Also, defects are prone to occur during the time-consuming thermal de-binding process.
This de-binding step is crucial since all organic materials are gasified and have to leave the green body through the small pores between the particles. If the amount of organic materials is high, and/or if the substrate is thick, the de-binding process will be very long, thus raising the production cost of the final products. The green body can also be damaged or even completely destroyed if gasification of the organic material during this de-binding step occurs at a too high rate or speed. Any damages in the green body will remain after sintering, thus lowering the strength of the final product. Additional problems such as, but not limited to, cracks caused by a differential thermal expansion of the matrix materials and the sacrificial materials have been reported as well. The de-binding step usually takes place at temperatures of about 200° C. to 500° C. Thermal expansion takes place when the green body is heated up to this de-binding temperature. Since different materials have different thermal expansion coefficients, stresses occur in the green body during this initial thermal cycle. These stresses can in turn cause cracks. The high temperature at which de-binding takes place also causes this technique to be useless for some applications. Catalysts e.g. for fuel cells can be destroyed or deactivated at these temperatures and some materials such as glasses start to sinter below the relevant de-binding temperature. In this situation there is a need for other methods for the production of porous materials.
Instead of spherical particles, an organic foam can be used as well as a template for the production of porous materials. This template is covered with the particles by passing a slurry through it. However the same de-binding problems exist for this production route and the control over the pore geometry is far less extensive than for other sacrificial materials. Moreover the struts of the final material are typically hollow when sponge templates are used.
A last drawback of the sacrificial materials technique is the cost of typical sacrificial materials such as, but not limited to, polymethylmethacrylate, polystyrene, starch and ammonium bicarbonate.
Partial sintering: the green body obtainable from a colloidal processing step can be sintered partially. A green body is in essence a powder packing with the ability to retain its shape. Consolidation of the solid form happens during sintering in which the individual particles are connected due to solid state diffusion. The material can be sintered to full density, meaning that all boundaries of all particles make contact with their immediate neighbours. The material can also be sintered to a lower density, in which case pores between particles will remain in the sintered end product. These pores have a random shape, and their size will depend on the extent of sintering and the particle size. If large or very large pores are needed, bigger particles have to be used. Bigger particles however have a detrimental effect on the global strength of the resulting material, since the largest grain determines the strength of a sintered piece if no other critical defects are present. The larger the grains, the lower the fracture strength. Furthermore the remaining pores usually have a random shape with some sharp edges which facilitate crack formation and growth, lowering the fracture strength even more. The inherent incomplete sintering aggravates this loss in strength even further. Porous particles or aggregates of porous particles can be used instead of individual solid particles to obtain the green body prior to sintering, e.g. in the production of porous hydroxyapatite.
Gas inclusion: by electrophoretic deposition, green bodies can be formed due to the application of an electric field. When aqueous media are used and high voltages are applied, electrolysis of water occurs. Thus the use of hydrogen and oxygen bubbles as a possible route for the production of porous materials has already been reported. When gas bubbles evolve at the deposition electrode they can be incorporated in the deposit, thus causing the formation of spherical pores. The voltage used offers some, but very limited, control over the size and amount of the bubbles formed. However a too high gas evolution hampers the electrophoretic deposition system due to a loss in effective electrode surface. Gas evolution may also cause premature loosening of the deposit from the electrode, or cause cracks in the green body.
Sol gel processing is a non-colloidal processing route that uses the gellation of ceramic precursors to form objects. These ceramic precursors may be organometallic compounds such as metal alkoxides which gel when exposed to water. The formed gel can subsequently be submitted to calcination and then sintered in order to consolidate the object. Since the gelled structure mainly consists of organic material and solvents, the calcination step must be performed with great care. The shrinkage of the structure during this step is substantial and the final product is usually only a fraction of the size of the initial gel structure. The use of emulsion droplets from a non aqueous emulsion has also been described as a method to obtain pores in a gelled structure, for instance in perovskite ceramic materials. These pores are retained after calcination and sintering. In a similar manner, hollow particles can be produced by coating emulsion droplets with a ceramic precursor gel followed by calcination. The major objections to sol gel processing are the high cost of the ceramic precursors and the troublesome calcination step. Shrinkage during the calcinations step complicates the prediction of the final dimensions, and handling of the gelled structure prior to consolidation can prove troublesome as well.
Polymerisation: ceramic objects can also be produced by incorporating ceramic particles into a polymer matrix, either by mixing a molten polymer with ceramic particles or by polymerising a suspension of ceramic particles in a monomer or pre-polymer. The resulting solid structure consists purely of polymer and ceramic particles. The polymer has then to be removed during a de-binding step. Afterwards the resulting ceramic particles can be sintered to consolidate the final product. If an emulsion of monomer and an other liquid is used, then a porous polymer ceramic composite can be obtained, the pores being retained during de-binding and sintering. The de-binding of the polymer matrix is the crucial step in this type of ceramic processing. The substantial amount of polymer causes this to be a slow process. Damages due to fast gasification also can occur. Thick objects are therefore difficult to obtain using this process. Furthermore substantial shrinkage occurs during this step, thus complicating the prediction of the final dimensions.
Polymer addition/stabilisation: in a similar fashion emulsions that are stabilized by a polymer binder matrix can be used as pore formers. For instance the use of polyvinyl alcohol for the preparation of porous high frequency single crystal capacitors has been described. Although the polymer content is usually lower than with a polymer matrix generated through polymerisation, the same problems remain.
Thus, there is a need in the art for an efficient process for the formation of porous materials, which reduces the risk of damages such as crack formation in the materials and/or which improves the cost effectiveness of the process.